In air conditioning systems for the aeration and conditioning of buildings or vehicles, the air generally not only has to be cooled, but also dehumidified since the air to be cooled often has such a high humidity that, upon cooling to the desired temperature, the dew point is fallen below. Hence in conventional air conditioning systems, dehumidification of the air accounts for a large part of electricity consumption.
One option for reducing the electricity consumption of air conditioning systems for buildings is the dehumidification of air by adsorption or absorption of water using a drying medium and regeneration of the drying medium laden with water by heating to a temperature at which the water is desorbed again. Compared to adsorption on a solid adsorbent, the advantage of absorption in a liquid absorption medium is that air dehumidification can be carried out with reduced equipment complexity and with less drying medium and that regeneration of the water-laden drying medium using solar heat is easier to carry out.
The aqueous solutions of lithium bromide, lithium chloride or calcium chloride hitherto employed as liquid absorption media in commercial air conditioning systems have the disadvantage that they are corrosive towards the metallic materials of construction typically employed in air conditioning systems and that they thus necessitate the use of expensive specific materials of construction. These solutions can additionally cause problems due to salt crystallizing out of the absorption medium.
Y. Luo, et al., Appl. Thermal Eng. 31 (2011) 2772-2777, proposes using the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate in place of aqueous solutions of lithium bromide for air dehumidification.
Y. Luo, et al., Solar Energy 86 (2012) 2718-2724, proposes using the ionic liquid 1,3-dimethyimidazolium acetate as an alternative to 1-ethyl-3-methylimidazolium tetrafluoroborate for air dehumidification.
US 2011/0247494 A1 proposes, in paragraph [0145], the use of trimethylammonium acetate or 1-ethyl-3-methylimidazolium acetate as liquid drying agent instead of aqueous lithium chloride solution. Example 3 compares water uptake from humid air for a series of further ionic liquids.
CN 102335545 A describes aqueous solutions of ionic liquids as absorption media for air dehumidification. The ionic liquids can contain the anions [BF4]−, [CF3SO3]−, [CH3COO]−, [CF3COO]−, [C3F7COO]−, [(CF3SO2)2N]−, [(CH3)2PO4]−, [C4F9SO3]−, [(C2F5SO2)N]− and [(CF3SO2)3C]−.
Commercially available ionic liquids generally comprise impurities which lead to substances that are odour-intensive or are injurious to health entering the dehumidified air upon dehumidification of air using the ionic liquid. In the desorption of water from ionic liquids which contain a basic anion, such as a carboxylate ion, odour-intensive decomposition products are formed which, in the event of a subsequent use of the ionic liquid for the dehumidification of air, enter the dehumidified air.
The problem was also observed that, in the case of purifying ionic liquids containing dialkylimidazolium ions by conventional purification processes, an at least partial decomposition of these ionic liquids occurs. This is apparent on occurrence of foul-smelling decomposition products in the ionic liquid obtained after the purification step.
The object of the present invention therefore consisted of providing a process for purifying an ionic liquid in which the aforementioned problems are minimized and ideally do not arise.